Nonvolatile memories (NVMs) are utilized in integrated circuits to provide storage of data when power is removed from the integrated circuit. It has been found that in NVMs, the threshold voltage (V.sub.ts) can vary over a wide range due to in-line process related stress and damage. It has also been found that the threshold voltage of one type of NVM known as a FLASH cell, in a mini array test structure is approximately 0.3 to 0.8 volts lower than the actual threshold voltage of the product cell. It has also been found that the threshold voltage for the reference cell can also be 0.3 to 0.8 volts below the threshold voltage (V.sub.ts) of the product. This difference causes the V.sub.ts of that given NVM device in the integrated circuit to be about a few tenths volt lower than the expected specification.
Accordingly, this V.sub.ts variation in reference cell of NVM product can create large column leakage currents that can cause a integrated circuit to function in an improper manner. In addition, it has also been found that this V.sub.ts variation is not uniform from NVM to NVM in the manufacturing process. This variation therefore causes product reliability issues in the NVMs. The V.sub.ts variation therefore requires that more complex synching and verifying circuitry be utilized with the NVM. The complex circuitry significantly increases the cost of the non-volatile memory. The V.sub.ts variation also causes extensive product testing and analysis, significantly increases the cost of manufacturing and oftentimes limits the fabrication flexibility of the particular circuit. Finally, this problem causes problems with the process control, as well as significantly decreasing product yield. When the process induced damage is large, the transconductance of the NVM can be seriously degraded and hence the device performance can be severely affected.
What is needed therefore is a protection structure for non-volatile memory and the like that solves the threshold voltage instability problem. This system should be easy to implement, should improve product reliability, should significantly improve the performance of the non-volatile memory. The present invention addresses such a need.